Tag Archives: PAMELA

Yesterday Sam Ting, the 1976 Nobel prize winner, announced the first results from his Alpha Magnetic Spectrometer (AMS-02) experiment. The results hint at a possible dark matter signal. But it remains only a hint.

The story begins with the observation that the visible stuff in the universe – stars, galaxies, and so on – are all made of matter. We see no evidence for large amounts of antimatter. However, small amounts of antimatter are constantly being created when high-energy cosmic rays scatter off particles in the interstellar medium, or when the intense electromagnetic field surrounding pulsars produce electron-positron pairs. So Earth is certain to be hit by antimatter particles, and in particular by positrons, that have been created by events within our galaxy.

Now, astrophysicists are interested in measuring the so-called positron fraction that hits Earth. (The positron fraction is the ratio of positrons to the total number of electrons and positrons.) If the main production mechanism for positrons is the scattering of high-energy cosmic rays off particles in the galactic disk, an assumption that until recently seemed quite reasonable, then the positron fraction would decrease with energy since there are other processes that generate high-energy electrons without accompanying positrons. In 2008, however, the PAMELA experiment measured a rise in the positron fraction between 10 GeV and 100 GeV. The Fermi satellite later confirmed this excess number of positrons, and it showed that the rise extended up to 200 GeV. So what is going on? Well, one explanation for the rising positron fraction is that positron generation by a few nearby pulsars could be the cause. But there’s another possibility.

If dark matter exists then sometimes, simply because there’s so much of the stuff, dark matter particles and antiparticles will meet and annihilate. In some models, dark matter annihilation can give rise to excess numbers of positrons. Furthermore, the dark matter signal must take a particular form: it will be isotropic (in other words, the positrons will come equally from all directions in space) and the rising positron fraction will have a sharp cut-off after a certain energy is reached (an energy that is determined by the dark matter particle mass, since the particles can’t give rise to positrons that are more energetic than themselves). So after the PAMELA and Fermi results there was hope that we might be seeing hints of a dark matter signal, but the data were not clear enough to draw any conclusions. It wasn’t even certain that the positron excess was real.

Enter Sam Ting’s AMS-02 experiment.

AMS-02 is a cosmic ray detector on board the International Space Station. The detector was put in place by astronauts on 19 May 2011, and since then it has detected 30 billion cosmic rays. It has been able to measure the positron fraction to higher energies than any previous detector and with a precision that is much, much better than anything that has gone before. Yesterday, Ting announced the results of an analysis of the first 10% of data from AMS-02.

An artist’s view of AMS-02 installed on the attached S3 location on the main truss of the ISS (Credit: NASA/JSC)

The first result is that the excess seen by PAMELA and Fermi is real: the positron fraction increases from about 5% at 10 GeV to about 15% at 350 GeV. That in itself is significant, and requires an explanation. Second, the excess seems to be isotropic: if it turns out to be truly isotropic then that would tend to disfavour pulsars as being the source of the excess. Third, there are suggestions – and these are nothing more than tantalising hints – that AMS-02 might be seeing the start of a cut-off.

The positron fraction as a function of positron energy. Compare the AMS-02 error bars with previous experiments! (Credit: AMS collaboration)

So are we seeing the effects of dark matter? As the years go by, and AMS-02 improves and extends the energy spectrum positron fraction yet further, astrophysicists might decide that the dark matter explanation is the only viable one. But at present it’s far too early to make any claims: the AMS-02 results announced yesterday are interesting, but nothing more.

As I’m sure you all know, the best model we have of the universe says that about 80% of its matter content is in some unknown form we call ‘dark matter’. (Most of the total mass-energy content is in some unknown form we call ‘dark energy’, but that’s another story.) Perhaps the best suggestion regarding the nature of dark matter is that it consists of WIMPs – weakly interacting massive particles. But what those WIMPs are, and precisely how heavy they are, remains unclear.

Normal matter forms only a small part of the mass-energy inventory of the universeCredit: NASA

There’s no general acceptance amongst the scientific community that dark matter particles have been directly detected, but there have been tantalising hints of WIMP detection in recent years. The Gran Sasso lab in Italy (which is home to the OPERA experiment, which recently observed the famous superluminal neutrino anomaly) is also home to the CRESST and DAMA experiments. Both experiments have made observations that are consistent with the detection of dark matter particles (it’s a strong claim in the case of DAMA). The Soudan mine in America is home to the COGENT experiment, which also saw events that are consistent with dark matter detection. Furthermore, the PAMELA cosmic-ray mission, which has been in orbit since 2006, has seen an abundance of positrons that some scientists have argued could be the product of dark matter annihilation.

In all the above cases the dark matter particles that are observed would be “light” particles – in other words, of relatively small mass.

A recent paper has added to the list of possible sightings of dark matter. The ARCADE ballon-borne experiment has been observing the sky in the radio spectrum, between 3-90 GHz, and has seen an excess of isotropic radiation. The paper suggests that this excess could be the result of WIMP annihilation: when WIMPs annihilate then many theoretical models suggest that they will generate pairs of electrons and positrons, which in turn will emit synchrotron radiation when they travel through magnetic fi elds. For this mechanism to explain the ARCADE results the WIMPs would need to have a mass in the range 10-20 GeV. This is not particularly massive; the WIMPs would be quite light.

So throwing all this evidence together can we conclude that dark matter particles are light? Well, no. The CRESST, DAMA and COGENT observations can all have other explanations, and in any case it’s difficult to reconcile all the data; whether the PAMELA excess can be attributed to annihilation of light dark matter particles has recently been called into question; and the ARCADE data could be the result of messy, poorly understood galactic astrophysics.